1. Field of the Invention
The present invention relates generally to high intensity discharge (HID) lamps, and more specifically to HID ballasts.
2. Description of the Prior Art
Ordinary incandescent lamps increase their resistance as their filaments approach operating temperature and are thereby current self-limiting for a given applied voltage. Gaseous discharge lamps, however, draw little current until they ignite and then show a negative resistance characteristic which requires external control. High intensity discharge (HID) lamps are powered by magnetic inductive devices, e.g., ballasts, that control the applied voltage and current.
Some conventional ballasts use "open core and coil" construction, in which a laminated steel or iron core has windings of exposed coils of wire. Typically, these open core and coil ballasts are used in outdoor and non-recessed ceiling applications, so thermal protection is not generally required. Where thermal protection is needed, automatic-reset thermal protection is provided, such as for the very largest ballast of a thousand watts.
Plastics and other insulators are used in open core and coil ballasts to electrically separate the conductor coils from the magnetic core. Typical insulations are rated for a maximum operating temperature of 180.degree. C. Transformers can also be constructed in open core and coil format, and their insulation systems range in maximum operating temperature from 90.degree. C. to 180.degree. C.
HID ballasts are also conventionally manufactured and marketed in encapsulated formats with automatically resettable thermal protection. These ballasts are mainly used for indoor recessed fixture applications, which requires automatically resettable thermal protection. The maximum operating temperature of ballasts using such encapsulants and insulation systems typically ranges between 90.degree. C. and 105.degree. C.
Core and coil ballasts can express certain rare modes of failure where the coils heat up very rapidly due to excessive currents. This can be caused by winding-to-winding shorts, excessive applied voltages, or other faults. Conventional automatically-resettable thermal protection, if built into a ballast, has not proved effective because the coils can rise to destructive temperatures before the automatically resettable thermal protectors can react. A thermal gradient develops between the source of heat and the detector that injects a reaction delay.
The prior art therefore allows destructive operating temperatures to be revisited many times because the system will automatically reset once the protection circuit has removed power and the system has had time to cool. While one such visit may not result in a catastrophic failure, many such cycles can almost be guaranteed to induce a failure which can either be benign or violent in nature. The violent failures must be avoided because they can cause serious personal injury or property damage, and can occur randomly without warning.
Simply lowering the trip temperature of automatic-reset thermal ballast protectors does not address the problem, because the trip temperature would have to be lowered into the normal operating temperature range of a ballast. Nuisance tripping would therefore occur. For example, an open core and coil ballast with a 180.degree. C. insulation system can be safely operated with internal temperatures that approach 165.degree. C. Such an insulation system will begin to rapidly degrade if temperatures are allowed to exceed 180.degree. C.
Ideally, a non-resettable thermal protector could be set to trip at 180.degree. C. A resettable thermal protector commonly needs a setting as low as 120.degree. C. to anticipate rapid temperature rises so that power can be interrupted before temperatures exceeding 180.degree. C. can be experienced. Therefore, a substantial down rating of power levels for open core and coil ballasts, and also transformers, would be required to use resettable thermal protection effectively, thus causing significantly larger and more costly implementations.
Non-resettable types of thermal protectors, based on melting wax or other materials, have been used in transformers, which are similar in character to ballasts. Such melting-material thermal protectors have proven to degrade over time when operated too near their trip temperatures. Failures are common at safe high operating temperatures because a downward shift occurs from the initial temperature trip-point.